Normally when I design suppression filters I use π-filters for data lines and T-filters for supply lines as supplies generally have a lower output impedance than data pins.
I now have the situation where a long cable carries both data and supply and it picks up (radiated) interference.
But is the output impedance of the supply relevant when suppressing this radiated interference? Should I keep π-filters on the data lines and T-filters on the supply lines? Or can I better switch to all π-filters (assuming the interference behaves as if coming from a high-impedance source).
The data cable carries a 5 V supply and 4 single ended UART data signals (115k2). These go into/come from a 'black box' for which I only have basic documentation. I don't know what's inside, I just have to work with it. I can't change anything on these signals I'm afraid. As mentioned, all data lines are fitted with π-filters and the supply line is fitted with a T-filter.
The π-filters (currently) consist of 2x 100 pF and a 300 Ω resistor. The T-filter consists of 2x ferrite bead (300 Ω @ 100 MHz) and 100 nF. These are of course to be further fine tuned.
The PCB contains a bridge sensing circuit with ADC. Even without modulation, the ADC gives out a different/wrong reading when the RF carrier is present. Experimenting with ferrite clamps on the supply/data cable and the cable connecting the bridge shows that both cables participate in picking up the noise and somehow transferring it to the ADC.
I tested for immunity at 10 V/m from 80 MHz to 1 GHz. Only for specific ranges of frequency does the ADC start to give out wrong readings. E.g. around 84 MHz, 205 MHZ and 500 MHz.
While researching the problems I experience I was wondering whether the impedance of radiated interference should be related to the circuit impedance or more to a 'general' (probably unknown) impedance.